dotfiles/example/microvm/mounts.nix

{ config, lib, ... }:

let
  inherit (config.microvm) storeDiskType storeOnDisk writableStoreOverlay;

  inherit
    (import ../../lib {
      inherit lib;
    })
    defaultFsType
    withDriveLetters
    ;

  hostStore = builtins.head (
    builtins.filter ({ source, ... }: source == "/nix/store") config.microvm.shares
  );

  roStore = if storeOnDisk then "/nix/.ro-store" else hostStore.mountPoint;

  roStoreDisk =
    if storeOnDisk then
      if
        storeDiskType == "erofs"
      # erofs supports filesystem labels
      then
        "/dev/disk/by-label/nix-store"
      else
        "/dev/vda"
    else
      throw "No disk letter when /nix/store is not in disk";

in
lib.mkIf config.microvm.guest.enable {
  fileSystems = lib.mkMerge [
    (
      # built-in read-only store without overlay
      lib.optionalAttrs (storeOnDisk && writableStoreOverlay == null) {
        "/nix/store" = {
          device = roStoreDisk;
          fsType = storeDiskType;
          options = [ "x-systemd.requires=systemd-modules-load.service" ];
          neededForBoot = true;
          noCheck = true;
        };
      }
    )
    (
      # host store is mounted somewhere else,
      # bind-mount to the proper place
      lib.optionalAttrs
        (
          !storeOnDisk && config.microvm.writableStoreOverlay == null && hostStore.mountPoint != "/nix/store"
        )
        {
          "/nix/store" = {
            device = hostStore.mountPoint;
            options = [ "bind" ];
            neededForBoot = true;
          };
        }
    )
    (
      # built-in read-only store for the overlay
      lib.optionalAttrs (storeOnDisk && writableStoreOverlay != null) {
        "/nix/.ro-store" = {
          device = roStoreDisk;
          fsType = storeDiskType;
          options = [ "x-systemd.requires=systemd-modules-load.service" ];
          neededForBoot = true;
          noCheck = true;
        };
      }
    )
    (
      # mount store with writable overlay
      lib.optionalAttrs (writableStoreOverlay != null) {
        "/nix/store" = {
          device = "overlay";
          fsType = "overlay";
          neededForBoot = true;
          options = [
            "lowerdir=${roStore}"
            "upperdir=${writableStoreOverlay}/store"
            "workdir=${writableStoreOverlay}/work"
          ];
          depends = [
            roStore
            writableStoreOverlay
          ];
        };
      }
    )
    {
      # a tmpfs / by default. can be overwritten.
      "/" = lib.mkDefault {
        device = "rootfs";
        fsType = "tmpfs";
        options = [ "size=50%,mode=0755" ];
        neededForBoot = true;
      };
    }
    (
      # Volumes
      builtins.foldl' (
        result:
        {
          label,
          mountPoint,
          letter,
          fsType ? defaultFsType,
          ...
        }:
        result
        // lib.optionalAttrs (mountPoint != null) {
          "${mountPoint}" = {
            inherit fsType;
            # Prioritize identifying a device by label if provided. This
            # minimizes the risk of misidentifying a device.
            device = if label != null then "/dev/disk/by-label/${label}" else "/dev/vd${letter}";
          }
          // lib.optionalAttrs (mountPoint == config.microvm.writableStoreOverlay) {
            neededForBoot = true;
          };
        }
      ) { } (withDriveLetters config.microvm)
    )
    (
      # 9p/virtiofs Shares
      builtins.foldl' (
        result:
        {
          mountPoint,
          tag,
          proto,
          source,
          ...
        }:
        result
        // {
          "${mountPoint}" = {
            device = tag;
            fsType = proto;
            options =
              {
                "virtiofs" = [
                  "defaults"
                  "x-systemd.requires=systemd-modules-load.service"
                ];
                "9p" = [
                  "trans=virtio"
                  "version=9p2000.L"
                  "msize=65536"
                  "x-systemd.requires=systemd-modules-load.service"
                ];
              }
              .${proto};
          }
          // lib.optionalAttrs (source == "/nix/store" || mountPoint == config.microvm.writableStoreOverlay) {
            neededForBoot = true;
          };
        }
      ) { } config.microvm.shares
    )
  ];

  # boot.initrd.systemd patchups copied from <nixpkgs/nixos/modules/virtualisation/qemu-vm.nix>
  boot.initrd.systemd = lib.mkIf (config.boot.initrd.systemd.enable && writableStoreOverlay != null) {
    mounts = [
      {
        where = "/sysroot/nix/store";
        what = "overlay";
        type = "overlay";
        options = builtins.concatStringsSep "," [
          "lowerdir=/sysroot${roStore}"
          "upperdir=/sysroot${writableStoreOverlay}/store"
          "workdir=/sysroot${writableStoreOverlay}/work"
        ];
        wantedBy = [ "initrd-fs.target" ];
        before = [ "initrd-fs.target" ];
        requires = [ "rw-store.service" ];
        after = [ "rw-store.service" ];
        unitConfig.RequiresMountsFor = "/sysroot/${roStore}";
      }
    ];
    services.rw-store = {
      unitConfig = {
        DefaultDependencies = false;
        RequiresMountsFor = "/sysroot${writableStoreOverlay}";
      };
      serviceConfig = {
        Type = "oneshot";
        ExecStart = "/bin/mkdir -p -m 0755 /sysroot${writableStoreOverlay}/store /sysroot${writableStoreOverlay}/work /sysroot/nix/store";
      };
    };
  };

  # Fix for hanging shutdown
  systemd.mounts = lib.mkIf config.boot.initrd.systemd.enable [
    {
      what = "store";
      where = "/nix/store";
      # Generate a `nix-store.mount.d/overrides.conf`
      overrideStrategy = "asDropin";
      unitConfig.DefaultDependencies = false;
    }
  ];
}